Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Both in the projection lens and in the illumination device there is a need to analyze the wavefronts propagating through the respective optical system during operation, in order e.g. to obtain information about the actually achieved optical effect of the individual optical components of the relevant optical system and the alignment thereof with respect to one another. In this respect, the principle of shearing interferometry is known, inter alia, in which, through the use of a diffraction grating, identical copies of the wavefront to be measured are generated in accordance with the different orders of diffraction and are brought to superimposition. Said diffraction grating is generally positioned at the focus or near the focus. Positioning the diffraction grating outside the respective focus position gives rise to periodic superimposition patterns, so-called multi-stripe interference patterns. In the case of pure defocusing, i.e. the diffraction grating is situated outside the focus and the wavefront is at least approximately an ideal spherical wave, an interference pattern having a regular, possibly curved, stripe pattern arises. In the case of local deviations from the ideal spherical wavefront (local gradients), the (local) frequencies and phase angles of the stripes vary.
The problem can occur here in practice that the relevant interference pattern is no longer able to be resolved by available camera-based detectors, for instance on account of an excessively high stripe density or an excessively high spatial frequency. This may be the case, for example, if the respective wavefront analysis is intended to be carried out at different positions or field points within the relevant optical system at which a markedly aspherical setpoint wavefront is present or where particularly curvilinear interference stripes with a wide spectrum of spatial frequencies and possibly high stripe densities may occur.
Regarding the prior art, reference is made merely by way of example to DE 101 09 929 A1 and WO 01/63233 A2.